The present invention relates to a printing apparatus and voltage control method, and more particularly, to a printing apparatus mounting a DC power source for driving an inkjet printhead on a head carriage substrate and a voltage control method.
Conventionally, two types of printing apparatuses are known: A thermal transfer method type, and an inkjet method type, the latter involving the discharge of ink onto paper or some other printing medium so as to form text or images. Inkjet printing apparatuses, which are widely used as data output means such as printers, copiers, facsimile machines and the like, print by discharging ink while moving the relative positions of the printing medium and the inkjet printhead. As a result, controlling the relative speeds of the inkjet printhead and the printing medium, as well as controlling the timing of the ink discharge and stabilizing the supply of power to the printhead are crucial determinants of the quality of the final printed output.
Inkjet printing apparatuses are broadly divided into two types, depending on the shape of the inkjet printhead being used: the so-called serial type, and the full-line type. Of these, the serial type, which is the more widely used, prints by discharging ink while moving the inkjet printhead.
In addition, among printheads that discharge ink, there are those that use the action of a piezoelectric transducer to discharge the ink and those that use instantaneous film-boiling of the ink to discharge the ink. Those printheads that boil to discharge the ink send an electric current to a heater provided adjacent to an ink flow path at an ink discharge orifice and utilize the thermal energy generated by the current to boil the ink so as to provide the discharge energy.
In order to maintain the quality of the printed data, it is important to maintain a stable supply of energy with which to discharge the ink and further to ensure that the ink is discharged under uniform conditions so as to obtain ink droplets of uniform size and shape. However, in printing, the duty ratio changes depending on the print data, so the number of heaters activated simultaneously at any given time varies as well. As a result, the drive conditions fluctuate due to voltage fluctuations caused by differences in current output by the power source and drop voltage differences caused by resistance in the power supply sub-system.
Conventionally, such ink discharge control is executed in such a way as to satisfy stable discharge conditions by refining the accuracy of the power supply output voltage and by reducing the loss along the power supply sub-system.
In order to facilitate an understanding of the present invention, a description is first given of the DC/DC converter that supplies power to the printhead in an arrangement related to this invention.
FIG. 11 is a block diagram of a voltage control circuit that forms a part of a DC/DC converter carefully studied as an example when this invention was made. Note that this example is not well-known to an ordinarily skilled person in this art.
As shown in FIG. 11, an input voltage (Vin) to the DC/DC converter that is supplied from a power supply unit (not shown in the diagram) is input to a switching element 201. A DC output converted by the switching element 201 and a diode 209 is output via an inductor 202 and is supplied as output voltage (VH-b) to the printhead, which is the load. A first condenser 203 is coupled to the DC side of the switching element 201 and a second condenser 204 is coupled to the AC side of the switching element 201, with the inductor 202 and the second condenser 204 forming a smoothing circuit 205.
The output voltage signal (VH-b) detected at the output terminal of the smoothing circuit 205 is divided by a first resistance R1 and a second resistance R2 at a voltage control circuit 206 and input to the negative (xe2x88x92) terminal of a differential amplifier 207 that forms the voltage control circuit 206 and is used for feedback control. An output signal (V refxe2x80x2) from the differential amplifier 207 that inputs both the electric potential achieved by voltage-dividing the reference voltage (V ref) by a third resistance R3 and a fourth resistance R4 and the divided voltage of the above-described output voltage signal (VH-b) becomes the output signal of the voltage control circuit 206, and controls the switching element 201 through a PMW gate drive circuit 208 so as to execute constant voltage control.
It should be noted that a fifth resistance R5 and a condenser C1 connected between the inverted terminal and the output terminal of the differential amplifier 207 are one example of a phase compensation circuit.
As thus described, the output voltage signal (VH-b) is feedback controlled so as to provide stable output voltage in the face of the output current fluctuations caused by changes in the number of nozzles simultaneously driven on the printhead which is the load.
In order to cope with recent technological advances, by which faster computers have made it easier to achieve image output of color image processing as well as image output from high-resolution digital cameras, inkjet printing apparatuses used as output apparatuses have had to simultaneously provide improved picture quality as well as faster printing speeds. Faster printing speeds can be achieved by increasing the ink discharge frequency and increasing the number of nozzles discharged simultaneously, and both faster printing speed and improved picture quality are achieved by increasing the volume of ink discharged per unit of time in droplet increments.
However, an examination of increasing the number of nozzles that simultaneously or substantially simultaneously discharge as a way of increasing printing speed reveals that, of those nozzles readied for simultaneous discharge, the necessity of discharging ink changes according to the image to be printed at that time. Thus, for example, whereas printing an entire page black requires that all the nozzles that can discharge ink actually do so, images with a low duty rate such as tables and the like require ink discharge from only a portion of all available nozzles.
As described above, when serial printing types of inkjet printheads print, that is, discharge ink, such printing is carried out using heat generated by the flow of an electric current through a heater.
With such an ink discharge method, the current required also increases proportionally to the increase in the number of nozzles that discharge ink simultaneously. Yet the required current is not always constant and uniform but varies continuously depending on the data sent to the printhead in proportion to the number of nozzles discharging ink.
In other words, depending on the image data transmitted from an external device, at an inkjet printing apparatus that forms an image, pattern or pattern character on a printing medium, the volume of ink droplets discharged per unit of time is determined by the amount of image data transmitted from the external device, and similarly, the amount of electric power consumed by the printhead is determined by the amount of image data per unit of time.
That is, the greater the amount of image data per unit of time, the greater the number of nozzles put into a state in which they are capable of generating a simultaneous ink discharge and the greater the amount of power consumed by the printhead. Conversely, the smaller the amount of image data per unit of time, the smaller the number of nozzles that simultaneously discharge ink and the smaller the amount of power consumed by the printhead. Similarly, the electric current that the DC/DC converter should deliver to the printhead is determined in proportion to the number of nozzles that are to simultaneously discharge ink.
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Next, in order to further facilitate an understanding of the present invention, a description is given of the power source that supplies electrical power to the printhead.
From the power source side, in order to minimize fluctuation in the output voltage with respect to fluctuations in current attendant upon the number of heaters simultaneously driven of the printhead that is the load, the stationary gain (K) of the voltage feedback control circuit can be increased. However, increasing the stationary gain (K) not only destroys stability during no-load operation but can also give rise to non-linearity in the PWM control sub-system.
Accordingly, since the stationary gain (K) cannot be increased for the reasons described above, the conventional voltage control is incapable of adequately coping with instantaneous fluctuations in current caused by, for example, the rapid ON/OFF action of the load (that is, the heaters), thus causing the output voltage transient fluctuation characteristic to deteriorate. As a result, conventionally a capacitor component typified by an electrolytic condenser is inserted into the output terminal to convert instantaneous current into average current, thereby minimizing output voltage drops due to instantaneous fluctuations.
More specifically, the power source of an inkjet printing apparatus, in which the drive conditions of the printhead that discharges ink droplets according to image data transmitted from an external device, is designed to supply a stable output voltage in the face of instantaneous load transitions including all printhead drive conditions. A description of how this stable output voltage supply is accomplished follows.
First, with respect to the instantaneous current fluctuations, swinging rapidly through the specified rated maximum current amplitude of a printhead drive that moves from a no-load state in which there is no ink discharge at all to a state in which ink is discharged from all the nozzles, instantaneous current fluctuations are averaged out and output voltage fluctuations minimized by a capacitor component inserted into the output terminal of the voltage supply circuit. The capacitor component may be an aluminum electrolytic condenser. Where adequate gain (K) cannot be obtained and the constant voltage control circuit cannot track the instantaneous current fluctuations, releasing the electrical charge stored in the electrolytic condenser maintains the necessary supply voltage to the printhead.
The problem with the conventional art is that, during load transition periods, as the load current swings from a no-load state (in which ink is not discharged) to a maximum current peak value (at which there is ink discharge from all the nozzles), with the conventional constant voltage control circuit, which is based on error amplification using the electric potential difference between the reference voltage and the output voltage, the DC/DC converter constant voltage feedback amount (that is, the differential amplifier differential voltage) experiences a delay and begins to decouple from the control of the constant voltage control circuit of the DC/DC converter, and the output voltage declines below a predetermined set voltage. In addition, the conventional attempt to solve the foregoing problem by correcting for the drop in output voltage requires greatly increasing the capacity of the output condenser, which interferes with efforts to make the DC/DC converter smaller and thinner.
Moreover, high-speed, high-resolution inkjet printing apparatuses continue to undergo increases in the printing width of the printhead as well as increases in the number of nozzles in the printhead, resulting in a trend toward increasing the number of nozzles that discharge simultaneously. Since such developments tend to increase the output current peak amplitude during load transition periods, some means other than increasing the capacity of the condenser for correcting for the drop in output voltage during load transition periods is desired.
Considering the above-described problems inherent in the conventional examples, in which an increase in the number of nozzles leads, for example, to an increase in the amplitude of the instantaneous current fluctuations used to drive the printhead of the inkjet printing apparatus, it is desirable to continuously provide a stable voltage and to minimize drops in that voltage so as to suppress reductions in output voltage generated in the time interval during which the constant voltage control circuit cannot track instantaneous fluctuations caused by the rapid ON/OFF action of the printing elements of the printhead, without increasing the size of the electrolytic condenser capacity that acts to retain the voltage at the output terminal of the DC/DC converter.
Accordingly, it is an object of the present invention to provide a printing apparatus and voltage control method that tracks instantaneous current fluctuations due to rapid ON/OFF operation of the printing elements of the printhead and supplies a stable voltage to the printhead, and that also restrains a voltage drop.
According to one aspect of the present invention, the above-described object is attained by providing a printing apparatus for printing a printing medium by moving a printhead including a plurality of printing elements, comprising: input means for inputting print data transmitted from an external device; a carriage, in which the printhead is mounted and a voltage control unit that supplies a controlled voltage for driving the plurality of printing elements of the printhead, for moving the printhead; counting means for counting a number of printing elements of the printhead to be driven based on the print data input by the input means; evaluation means for evaluating an extent of a load of a succeeding print cycle to be applied to the printhead based on a count result by the counting means; and control means for inputting an evaluation signal indicating an evaluation result by the evaluation means to the voltage control unit, and controlling the voltage based on the evaluation signal.
Preferably, the above-described evaluation means evaluates the extent of the load in multiple stages and includes at least a light load detection circuit, a heavy load detection circuit and a medium load detection circuit.
In addition, preferably the voltage control unit is a DC/DC converter, with the DC/DC converter including a differential circuit that detects a signal change in the evaluation signal and an adder circuit that adds an output from the differential circuit to a reference voltage of the DC/DC converter. Moreover, preferably the DC/DC converter further comprises a time constant circuit for dampening detection of a signal change by the differential circuit.
Preferably, the above-described printhead is an inkjet printhead that prints by discharging ink, comprising (1) a first inkjet printhead that discharges black ink, (2) a second inkjet printhead that discharges cyan ink, (3) a third inkjet printhead that discharges magenta ink and (4) a fourth inkjet printhead that discharges yellow ink. Preferably, the inkjet printhead comprises an electrothermal transducer that generates thermal energy to be added to the ink in order to discharge ink.
Moreover, preferably the counting means counts each black data, cyan data, magenta data and yellow data color component. In such a case, preferably the evaluation means outputs a black-and-white evaluation signal based on the black data and a color evaluation signal based on the cyan data, the magenta data and the yellow data, and the voltage control unit supplies both a drive voltage for black-and-white printing and a drive voltage for color printing.
According to another aspect of the present invention, the foregoing object is achieved by providing a voltage control method for controlling a drive voltage for driving a printhead having a plurality of printing elements mounted on a printing apparatus for printing on a printing medium, comprising the steps of: inputting print data transmitted from an external device; counting a number of printing elements of the printhead to be driven based on the input print data; evaluating an extent of a load of a succeeding print cycle to be applied to the printhead based on the count result; and inputting an evaluation signal indicating the evaluation result to a voltage control unit for supplying a controlled voltage so as to drive the plurality of printing elements, and controlling the voltage based on the evaluation signal.
In accordance with the present invention as described above, in order to control a drive voltage for driving a printhead having a plurality of printing elements mounted on a printing apparatus for printing onto a printing medium, print data transmitted from an external device is input and, based on that input print data, the number of printing elements of the printhead to be driven simultaneously is counted and, based on the results of that count, the extent of the load of the next single print cycle to be applied to the printhead is evaluated, an evaluation signal indicating the evaluation results is input to a voltage control unit that supplies a controlled voltage for driving the plurality of printing elements of the printhead, and a compensation voltage to compensate for a voltage drop caused by the load when driving the printhead is applied based on that evaluation signal.
The invention is particularly advantageous insofar as it can track instantaneous current fluctuations due to rapid ON/OFF operation of the printing elements of the printhead and supply a stable voltage to the printhead, as well as restrain a voltage drop.
Other objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.